There are useful scanning ophthalmic diagnostic devices that build up a map of measurements of the retina of a patient. This can be accomplished using different imaging modalities such as optical coherence tomography (OCT) and scanning laser polarimetry (SLP). Examples of two such commercial devices are Stratus OCT and GDx, both manufactured and sold by Carl Zeiss Meditec, Inc. (Dublin, Calif.). In the time (usually a few seconds) required to build a useful map, the patient's gaze can shift, causing the retinal image to move from the point of view of the ophthalmic device. Retinal tracking has been used to follow and correct for eye motion. In this case, the scanning beam is adjusted to compensate for movements and collect data from the optimal location. For example, systems have been described that detect apparent motion of the retina using a tracking beam and move mirrors in the imaging path to provide a stabilized OCT image (see for example U.S. Pat. Nos. 6,736,508, 6,726,325 and 6,325,512). Additionally U.S. Pat. No. 7,805,009 describes the use of a line scan ophthalmoscope to monitor the position of the eye and apply a correction to the OCT scanner. The GDx instrument takes a different approach; instead of stabilizing the retinal image, GDx registers the 32 successive retinal images, each imaged using a different illumination polarization, to make the polarimetry map. Even with tracking or registration, there are however, situations that cause some of the measurement data to be unusable. If there is motion or a blink that occurs before the tracker has determined that there was motion, data collected up to that point would likely contain motion artifacts and would likely be unusable for measurement and diagnostic purposes. If the patient rapidly shifts their gaze in a saccadic motion, the data taken before the tracker had a chance to compensate would be measured at the wrong location on the retina. The methods described above do not address this missing data problem. For example, the method described in U.S. Pat. No. 7,805,009 uses a line-scan imager to monitor the retina for motion. Each line of image data from the line scan imager is compared with a reference image to find the displacement of the subject that can be used to correct the OCT image data. This has the limitation that it cannot handle large and sudden movements such as saccades. Also none of the methods described above account for motion of the subject along the axial direction.
Prior art tracking systems, repeat one line of the OCT measurement sequence until sufficient data is collected while the retinal image is reasonably stable, but they do not continue scanning, determine that there was motion and then go back to retake data missed due to eye motion. Also prior art instruments do not provide the user a way to control the tracking and to obtain the data in an efficient way in the shortest possible time. Another concern with the current tracking devices is that the time for an acquisition is considerably longer than without tracking because the tracker keeps retrying to acquire the data if the eye is not stable.
It is therefore an object of the present invention to provide systems and methods for retinal tracking that overcome the above identified limitations.